In principle, a single web server can provide all functionality to allow web browsers (clients) to access a web site. However, in recent years web sites have grown more complex, and so have web server systems.
For some years already it is customary to store the web site's content in a database, from which HTML pages are generated on the fly in response to a request. The system for serving web pages thus comprises a web server combined with a database backend. A different system may provide templates that indicate the look and feel of the website.
For large-scale web sites, in order to have redundancy and/or geographic distribution, distributed servers and database systems may be used, which adds further complexity to the design.
In recent years, more and more regulation relevant for web sites has appeared. National or trans-national regulation may stipulate if and how a web site may handle information related to people browsing the web. A common motivation of the regulation is to safeguard privacy of the people surfing the World Wide Web. For example, recent European regulation specifies the conditions for web sites to store so-called “cookies” (persistent information related to browsing) using a user's web browser.
Since the Internet does not generally stop at national borders, a web server administrator has to deal with many different regulations. It is naturally difficult for a web server to adhere to all regulations that may be relevant to any of its users. Accordingly, there is a need to handle personal or privacy related information in a structured fashion.
An article by R. J. Schloss, “Novel business uses of independently created hyperlinks in the World Wide Web: basic mechanism and examples” (Proceedings of the annual Hawaii international conference on system sciences, 1996) pp. 137-146, discloses an advisory server which, when provided with an URL of a page visited by a browser, can provide additional data on said visited page. The browser can then present the additional data along with the visited page. A disadvantage of this system is that it must be supported by the browser—in other words, the browser must, at its own initiative, explicitly request the additional information from the advisory server, using the Web Advisory Transmission Protocol (WATP). This requirement severely limits the use of the advisory servers, in particular for commodity devices with “off-the-shelf” browser software, such as connected appliances (so-called “Internet of Things” IoT devices).